An apparatus for magnifying and projecting an image by sequentially scanning a light valve with a plurality of different colors of. light has been known from JP 6(1994)-319148 A. The apparatus includes first, second and third polygonal prisms that are arranged coaxially and shifted relative to one another by a rotation angle of 30 degrees with respect. to the rotation axis. The polygonal prisms are rotated, so that the different colors of light entering the respective prisms are refracted to scan the light valve sequentially.
As described above, the rotating polygonal prism is used to deflect light incident thereon by refraction. Thus, scanning linearity depends on the number of surfaces of the prism. To improve the linearity, it is necessary to increase the number of surfaces of the prism. However, a predetermined size of each surface of the prism for receiving light has to be ensured. Therefore, an increase in the number of surfaces inevitably leads to an increase in size of the prism. Moreover, to rotate a large prism, a motor or the like, acting as a rotation system, requires a large rotation torque. When a prism having edges is rotated, the chance of wind resistance occurring increases, and thus a larger rotation torque is needed. These bring about a rise in the cost and in the power consumption. In addition, the occurrence of wind resistance causes noise, which becomes a major obstacle to showing pictures.